For example, a cellular type automobile/portable radio telephone system has been known as one of mobile communication systems.
For example, as shown in FIG. 9, this type of system comprises a control station CS connected via a wired line CL to a stational exchanger EX of a public station network, a plurality of base stations BS1 to BSn connected via wired lines CL1 to CLn to the control station CS, and a plurality of mobile stations MS1 to MSm. The respective base stations BS1 to BSn constitute radio zones E1 to En called cells in different areas within a service area of the system. Different radio channel groups are assigned in advance to the radio zones E1 to En. These radio channel groups are repeatedly assigned to other cell groups located at a sufficient distance, thereby effectively utilizing the radio channels. FIG. 10 shows an example of this structure, and radio channel groups F1 to F13 are assigned to 13 cells constituting each cell group.
Regarding the mobile stations MS1 to MSm, each station is connected to the base station of the radio zone, in which the present station is located, via an unoccupied radio channel of the radio channel group assigned to this radio zone, and the base station is in turn connected to the stational exchanger EX via the control station CS. In this state, a call can be made with a wired telephone connected to the stational exchanger EX or another mobile station. If the mobile station, MS1 to MSm, has moved to another radio zone during a call, the radio channel is switched to a radio channel assigned to the radio zone of the destination of the mobile station and the call is continued. Specifically, as long as the mobile stations MS1 to MSm are present within the service area of the system, a call can be made between each station and another telephone via the base station of the radio zone (cell) where the station is located.
This cellular radio telephone system, however, has the following problem. In the cellular radio telephone system, a mobile station is permitted to make a call anywhere in the service area. Thus, when the user is present, in particular, outdoors, the feature of the system can be exhibited to a maximum. However, the charging rate of the call fee of the cellular radio telephone system is generally higher than that of the public telephone network using wired lines. Accordingly, when the user makes a call in the home or office, it is economical to use a wired subscriber line of the public telephone network, rather than using the cellular radio telephone system. For this purpose, however, the user must deliberately choose between the use of the mobile station of the cellular radio telephone system, i.e. the portable telephone, and the use of the wired telephone installed in the home or office, each time he makes a call. This is very troublesome.